1. Field of the Invention
This invention relates to a micellar slug suitable for use in a micellar drive for recovering oil from subterranean reservoirs. More specifically, it relates to a micellar slug, suitable for use in a micellar drive for recovering oil from subterranean reservoirs, which is capable of forming micro-emulsions at a high salt concentration and which has a large capability to maintain the micro-emulsions against change in the composition of the micro-emulsions due to the fact that the micellar slug is capable of forming micro-emulsions in a wide composition range.
2. Description of the Prior Art
So-called "primary recovery" methods, including pumping methods, can recover only a portion of the petroleum or crude oil ("oil") from subterranean reservoirs and leave substantial amounts of oil in the subterranean reservoirs. So-called various "enhanced oil recovery" (EOR) methods have been proposed to recover the remaining large amounts of oil from the subterranean reservoirs.
Of these EOR methods, the recent "micellar drive" methods are to be noted. According to these methods, a micellar slug, that is, a clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates, or fuel oil, is injected under pressure into the subterranean reservoirs for the recovery of oil in the subterranean reservoirs. These micellar drive methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants including anionic-, nonionic-, and cationic-type surfactants can be used alone or in any mixture thereof in the formation of micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.
The surfactants used in the formation of a micellar slug must be available at a low cost, while still displaying the required performances, since a large amount of a micellar slug is necessary to recover oil from subterranean reservoirs. In addition, the numerous oil production wells or oilfields present in the world include subterranean reservoirs with a variety of properties. The available water also varies from soft water containing no substantial amounts of inorganic salts to brine containing large amounts of inorganic salts and polyvalent metallic ions. Thus, the surfactants used in micellar slugs should also have good thermal stability, salinity tolerance, and hard-water resistance.
It is believed in the art that petroleum sulfonate is an optimum surfactant usable as an injection fluid in a micellar drive due to its availability and low cost. However, a problem exists in that petroleum sulfonate has an unsatisfactory salinity tolerance and hard-water resistance, and, therefore, can be applied only for certain oilfields. For this reason, various attempts have been made to improve the properties of petroleum sulfonate by using petroleum sulfonate together with other surfactants. However, micellar slugs having the desired properties and a satisfactory cost have not been obtained.
We have proposed, in U.S. patent application Ser. No. 415,840 filed on Sept. 8, 1982, the use of an alpha-olefins sulfonate as a surfactant capable of providing a micellar slug having an interfacial tension decreasing capability equal to or more than that of a micellar slug containing petroleum sulfonate and having an excellent salinity tolerance and hard-water resistance and a relatively high viscosity. However, the micro-emulsion formed when a micellar slug is prepared by using an alpha-olefin sulfonate, is likely to be destroyed when injected under pressure into subterranean reservoirs due to the fact that the composition range within which a micro-emulsion can be formed is not very wide. Accordingly, this micellar slug should be further improved from a practical viewpoint.